(Reuters) – Aptera, the California startup best known for developing a space age-looking, three-wheeled car, has closed its doors after failing to raise the funds needed to put its ultra-efficient vehicles on the road. The company had received a conditional offer for a $150 million loan from the U.S. Department of Energy, but failed to [...]

AltaRock received another $26.5 million in grants from the Department of Energy today for two new geothermal drilling projects — despite claims that its drilling project in the California Geysers could cause earthquakes.

The New York Times ran a story in June comparing Altarock’s California project to a 2006 project in Basel, Switzerland, where an earthquake damaged buildings and frightened residents with a loud roaring noise. Aftershocks followed for several months.

AltaRock — which has raised over $30 million from Khosla Ventures, Kleiner Perkins, GreatPoint Ventures, Google.org and others to provide clean energy by extracting heat from rocks — wasn’t behind the Basel project and disputed the Times’ claims. But the story prompted a review by the Department of Energy and the Bureau of Land Management, which said that although AltaRock could keep drilling, it couldn’t fracture any rock in California until the agencies decided it was safe.

Capital is tight for clean tech, and it’s not just because of the economy — investors are also being asked to back some pretty long-term technologies, and not everyone wants to be first to assume the risk.

Consider Potter Drilling, which has the potential to solve a very big problem. With enough financial backing, it could supply enough clean energy to equal 2800 times the current consumption of the U.S. — if it can drill enough 10-kilometer-deep holes to extract the heat that’s trapped in the rocks below the surface of the earth. Every town could have its own hole, says CEO Jared Potter — and it’s own little power plant.

It’s a tantalizing picture, but now, drilling a well that’s only 30,000 feet (9.1 kilometers) deep costs over $200 million with today’s technology. Asking investors to bet on a vision has been tough.

Backed by over $30 million in venture capital and a $6.25 million DoE grant, AltaRock Energy has been drilling in Northern California geysers to establish pathways to hot rock, which it wants to inject with water to capture heat that would generate electricity.

Yesterday, AltaRock suspended the project, citing “physical difficulties” in the drilling of its first well, “resulting from geologic anomalies particular to the formation underlying this well location.” It claims to have not given up, instead saying that it’s evaluating other locations for drilling with a reminder that it’s filed 20 patents.